The present invention is related to a cathode ray tube, and more particularly cathode ray tube wherein the divergence angle of an electron beam emitted from a cathode of an electron gun is decreased by a magnetic field, and the electron beam is accelerated to improve the current density of the electron beam passing through the electron beam passing holes of a control electrode.
Generally, a cathode ray tube comprises a panel having a shadow mask frame assembly therein, and a funnel having a neck portion which houses an electron gun. The electron gun mounted in the neck portion of the funnel as shown in FIG. 1, comprises a triode as the source of an electron beam. The triode comprises a cathode 11, a control electrode 12 and a screen electrode 13, and a main lens including a focus electrode 14 and final accelerating electrode 15, and a bead glass 16 supporting the whole structure.
In the electron gun of a cathode ray tube constructed as above, as a predetermined voltage is supplied to each electrode, a cathode lens is formed between a cathode 11 and a control electrode 12, a pre-focus lens is formed between a screen electrode 13 and a focus electrode 14, and a main lens is formed between a focus electrode 14 and a final accelerating electrode 15. Accordingly, the electron beam emitted from cathode 11 is preliminarily focused and accelerated in the cathode lens and pre-focus lens, and then finally focused and accelerated by the main lens to proceed toward a fluorescent layer at a high speed.
However, as shown in FIG. 2, since tile diameter of an electron beam passing hole 12a of control electrode 12 is smaller than that of a base metal 11a of cathode 11, the electron beam emitted from electron-emitting material of cathode 11 does not smoothly proceed to electron beam passing hole 12a, and a considerable amount of the electron beam collides with control electrode 12 and is eliminated. Also, since 0 V and 400 V are supplied to control electrode 12 and screen electrode 13, respectively, the divergence angle of some of the electron beams emitted from the electron-emitting material is enlarged by the magnetic field formed between control electrode 12 and screen electrode 13. Therefore, tile electron beam is affected by a large spherical difference when passing through the cathode lens and pre-focus lens.